1. Field of the Invention
The present invention generally relates to the field of electronics. More particularly, the invention relates to bulk acoustic wave (BAW) resonators.
2. Background Art
Because of their small footprint, low profile, and high performance, bulk acoustic wave (BAW) filters are increasingly utilized to provide radio frequency (RF) filtering in mobile electronic devices, such as cellular phones, as well as other types of electronic devices. BAW filters can include a number of BAW resonators, where each BAW resonator typically includes a layer of piezoelectric material, such as aluminum nitride, sandwiched between upper and lower electrodes. When an electric field is applied across the upper and lower electrodes of the BAW resonator, the electric field can cause the layer of piezoelectric material to vibrate. As a result, the piezoelectric material can generate a number of allowed modes of acoustic wave propagation, which include a desired longitudinal mode. However, unwanted excitation of energy in modes of wave propagation that have high energy loss, such as lateral modes, can cause a significant loss of energy in a BAW resonator and, thereby, undesirably lower the BAW resonator's quality factor (Q).
Conventional approaches to reducing energy loss in a BAW resonator include shaping the profile of the resonator such that the energy is best contained and controlled in a desired longitudinal mode. In one conventional profile shaping approach, a shaped region can be provided close to the edge of the BAW resonator, which is a region of high energy loss, to reduce the amount of energy that is excited in lossy modes of wave propagation in the BAW resonator. However, the shaped region provided in this conventional approach can also introduce additional unwanted modes, such as lateral modes contained within the shaped region, which can cause energy loss in the BAW resonator.